In accordance with an improvement in the functions, increase in the number of components of mounted circuit devices is inevitable in mobile apparatus such as a personal computer, a portable telephone, a fixed telephone, and a PDA, audio apparatus such as a television set and an audio player, information terminal apparatus such as an information management terminal on the point of sale, and home-use electric appliances such as a cleaner, a refrigerator, an electric rice-cooker, and an air conditioner. On the other hand, products tend to have smaller weight, thickness, length, and scale, so that a breakthrough in the component construction and the mounting method is needed. In order to solve this problem, it is effective to form an electric circuit also on the housing for saving space and reduction of the number of components. In particular, reduction of the mounted antenna volume is demanded in order to meet the multilateral wireless and multi-resonance communication of mobile apparatus such as a personal computer or a portable telephone. If an antenna can be formed directly on a housing, a large volume reduction effect can be expected. For example, a wireless apparatus is proposed in which an incorporated type antenna is formed on a housing (Patent Document 1).
A technique of forming an electric wiring on a resin member is generally referred to as the MID (Molded Interconnect Device) method. The production method is roughly classified into a one-shot method and a two-shot method depending on the number of times for injection molding of a resin composition. Also, there are various ones in the two-shot method. A representative one thereof is the following method. That is, a primary molded article having a predetermined three-dimensional shape is formed by injection molding of a resin composition. Next, after surface roughening of the surface thereof in accordance with the needs, a secondary molded part is formed to cover a region of the surface of this primary molded article other than the region where circuits are to be formed (which may hereafter be referred to as “regions other than the circuits”) by insertion molding, thereby to obtain a so-called two-color molded article. Next, a state is made in which a catalyst for non-electrolytic plating is imparted to the entire surface of this two-color molded article. Then, by performing non-electrolytic plating, a conductive layer that will be a circuit having a predetermined pattern is formed in a region on the surface of the primary molded article to which the catalyst has been selectively imparted, thereby to obtain a MID (Patent Document 2). This method has a long production lead time by a complex molding and treating process, and necessitates fabrication of a plural number of exclusive-use molds, thereby increasing the production costs. Also, in the case of changing the circuit design, change must be made starting from a modification of the molds, thereby providing a poor degree of freedom in designing. Also, the costs will rise, so that this method is not practical as a method of forming a wiring on a housing that often tends to have a complex shape.
On the other hand, a circuit made by printing a conductive paste by the screen, pad, gravure, flexo, or the like printing on a substrate made of an insulating material and having various shapes such as a film or a plate is used in various scenes because of having a small weight and being capable of forming printed circuits in various places in various forms. A circuit made from conductive paste alone has a disadvantage of having a considerably higher specific resistance compared to a conventional copper foil or aluminum foil; however, this disadvantage is solved by performing plating on the conductive paste. In recent years, there is a demand for adopting an insulating substrate such as resin having a low heat resistance for a printing substrate. The resin that is commonly used as a housing material of various appliances and apparatus has a low heat deformation temperature. For example, the PC/ABS resin (grade MB2215R for portable telephone housing, manufactured by Mitsubishi Engineering Plastics Co., Ltd.) has a low heat deformation temperature of 92° C., so that a low-temperature curable type conductive paste that is cured by being heated at a lower temperature than this is demanded.
Patent documents 3 and 4 disclose a conductive paste that is suitable for use as a plating underlayer. The curable system used herein, which consists of polyester or polyurethane and isocyanate blocked with an oxime necessitates a heating treatment at about 150° C. in curing, so that it is difficult to apply this to a substrate having a low heat resistance such as polycarbonate. Also, it is possible to perform electrolytic plating that is performed in an acidic plating solution having a temperature around an ordinary temperature; however, exfoliation of the coating film may occur in a non-electrolytic plating process that is exposed to a high temperature and a high alkalinity.
Patent document 5 discloses a conductive paste of thermoplastic type that does not contain a curing agent and a conductive paste having a low curing temperature lowered by compounding a curing catalyst, as a conductive paste that can form a conductive coating film by a heating treatment at 80° C. and for 30 minutes. However, both of these use mainly a polyester resin or a urethane-denatured polyester resin as a binder resin, so that, when the plating solution has a high temperature and a high alkalinity, the binder resin may in some cases be deteriorated to cause exfoliation of the plating immediately after the plating is performed, so that this may be hardly usable as a conductive paste for a plating underlayer.
(Patent Documents)
                1. Japanese Patent Application Laid-Open (JP-A) No. 2008-160684        2. Japanese Patent Application Laid-Open (JP-A) No. 145583/99        3. Japanese Patent Application Laid-Open (JP-A) No. 293213/96        4. Japanese Patent Application Laid-Open (JP-A) No. 194768/97        5. Japanese Patent Application Laid-Open (JP-A) No. 2006-252807        
In view of the above circumstances of the related art, an object of the present invention is to provide a conductive paste being curable by a heating treatment at a low temperature of about 80° C. which is lower than in conventional cases, being capable of withstanding any of the processes of electrolytic plating and non-electrolytic plating, being excellent in printability, and being capable of forming an electric wiring having a further better conductivity by performing plating.
In order to achieve the above described object, the present inventors have made eager analyses and studies, and as a result have found out that a specific conductive paste containing a conductive powder, an organic resin, a curing agent, and an organic solvent has a good conductivity and physical property as a coating film while having a low-temperature curable property, and is excellent in plating property and a printing property, thereby arriving at the present invention.